In recent years, along with trends for high speed and large sizing of industry, rubber components to be used in such industry tend to be required to have an increasingly high level of durability. Especially, the trends for high speed and large sizing tend to bring about an increase in the operation temperature of the machine and apparatus, whereby a high level of heat resistance is required for the rubber components, and at the same time, due to an increase in the temperature of a lubricating oil, an improvement in oil resistance is required for the rubber components. Further, for the use in a severe environment such as in a cold area due to an expansion of the industrial activities, the demand for cold resistance has increased for the rubber components.
JP-B-59-14498 discloses that a rubber composition comprising a vulcanizing agent and a copolymer made of ethylene, vinyl acetate, an acrylate and a monoepoxy monoolefin compound such as glycidyl methacrylate, as a crosslinking site monomer, is excellent in the oil resistance, heat resistance and weather resistance.
With such a composition, it is observed that the balance of the oil resistance, heat resistance and weather resistance, is good. However, in view of the severe conditions in use as mentioned above, a further improvement has been desired in the balance of the oil resistance, cold resistance and heat resistance.
Further, such a composition can be vulcanized by means of a vulcanization system such as a fatty acid soap/sulfur, a polyamine type, or a carboxylic acid and its ammonium salt, and the mechanical properties are good. However, it has a drawback that the vulcanization speed is slow.
Namely, in order to obtain the desired physical properties, it is common that after carrying out usual vulcanization, post vulcanization is carried out. If this post vulcanization can be omitted completely or if the time required for the post vulcanization can be shortened, such will be industrially very significant, and the demand for a rubber composition improved for such a purpose, has rapidly increased.
It is known by JP-A-50-45031 that an elastomer composition containing an alkyl acrylate or ethylene/an alkyl acrylate as the main component and employing alkyl monoester of a butene-1,4-dionic acid as a crosslinking site monomer, has a good vulcanization speed.
On the other hand, as a method for producing such a rubber, it is common to employ a method wherein a rubber latex is firstly obtained by polymerization, and an acid or a salt is, for example, added thereto for coagulation, followed by dewatering and further drying.
JP-B-3-29085 discloses a continuous production method for a dry rubber wherein a mixture comprising a rubber latex containing a polyvinyl alcohol as an emulsifier, and a boron compound and a compound containing a sulfate group, as coagulating agents, is dewatered and supplied to a drying apparatus, so that coagulation, dewatering, drying, etc., can be carried out continuously and in good yield.
However, in a case where the rubber latex is a latex of a carboxyl group-containing rubber, the coagulation state tends to be inadequate, whereby there will be a drawback such that it is not durable against mechanical pressing, the yield of the dry rubber tends to be inadequate, and the obtained dry rubber tends to undergo scorching in the kneading step.
The present invention is to solve the above problems and to provide a method for producing a carboxyl group-containing rubber having good vulcanization characteristics and exhibiting good resistance to scorching, continuously and in good yield.